In a system with only a few AGVs, the failure of any one of the AGVs will not cause a significant traffic congestion issue. Moreover, the failed AGV can be quickly replaced by back up ones. Hence, such a small scale AGV application system can be easily managed. However, given the increasing number of large scale AGV application systems where a significant number of AGVs share the limited number of travel routes, the failure of any one of these AGVs will cause serious traffic chaos. Hence, for this reason, considering a complete investigation of the reliability issues of

all AGV components and subassemblies is important not only to ensure the high reliability and availability of AGVs and their success of delivering prescribed tasks but also to optimise their maintenance strategies and minimise traffic chaos. The reliability issues of the whole AGV system are investigated through assessing the reliability of a typical AGV transport system in this paper, where the capability to

consider mission analysis of AGVs is shown. The novelties of this paper can be summarise as the following: an effective reliability assessment of AGVs using combined FTA and PNs has been proposed. Using the techniques developed,

the critical phases in the AGV mission can be identified and their failure probability can be obtained. The PN simulation is found to be an efficient and adaptable method to analyse the reliability of complex AGV systems undertaking various tasks

The remaining part of the paper is organised as follows. In Section 2, the reliability modelling methods are discussed with the AGV application system being the focus of Section 3. Section 4 covers the AGV system reliability model generation and the AGV mission analysis is covered in Section 5.The simulation method adopted and the results are presented in Section 6 with the conclusions in the final section.

2 Reliability modelling

One of the most commonly used reliability methods, widely adopted in industrial practise, is fault tree analysis (FTA). This method allows a system failure mode to be expressed in terms of the interactions of its components. Moreover, with the aid of FTA, the probability of system or mission failure can be computed via Boolean logic calculations. This method has been adopted to evaluate the subsystem level failures for the AGV system

When the system is large and complex, or the mission performed is made up of many phases, FTA can become inaccurate and computationally expensive. In such cases, alternative reliability modelling methods may be better suited to performance analysis, one such technique being Petri nets (PNs), developed by Petri. Similar to FTA, PNs provide an intuitive graphical representation of the system being modelled allowing for reliability investigation. The PN method is a direct bipartite graph which consists of four types of symbol: circles, rectangles, arrows and tokens.

Circles represent the places, which are conditions or states such as mission failure, phase failure or component failure; rectangles represent the transitions, more abstractly actions or events which cause the change of condition or state. It should be mentioned that if the time for completing the transition is zero, the rectangle is filled in, otherwise it is hollow; arrows represent arcs which are connections between places and transitions. Arcs with a slash on and a number, n, next to the slash represent a combination of n single arcs and the arc is said to have a weight n. No slash always means that the weight is one; and small filled in circles represent tokens

which carry the information in the PNs. The tokens move via transitions as long as the enabling condition explained below is satisfied, which gives the dynamic properties of the PNs. The marking of a net at any particular time gives the state of the system being modelled at that time.

Figure 1 shows an example net where movement of tokens have occurred after a time period, hence the tokens make a transition through the net. The net has two input places (represented by circles) and one output place (drawn as a circle) connected by a timed transition (hollow rectangle) with a time delay t. There is one token and three tokens (represented by small filled in circles) in each of the two